Method for producing porous ceramic article

ABSTRACT

A method for producing a porous ceramic article involves a step of kneading raw materials including a raw material for a ceramic and a processing aid, to prepare a body, a step of forming the body, to prepare a ceramic formed product, a step of drying the formed product, to prepare a ceramic dried article, and a step of firing the ceramic dried article, to preparing a porous ceramic article, characterized in that the processing aid is a starch having been subjected to a crosslinking treatment or a material comprising a starch having been subjected to a crosslinking treatment and a foamed resin. The method allows the production of a porous ceramic article which is suppressed in the deformation during drying and is excellent in dimensional accuracy.

TECHNICAL FIELD

The present invention relates to a method for producing a porous ceramicarticle. More particularly, the present invention relates to a methodfor producing a porous ceramic article excelling in dimensional accuracyby suppressing deformation during drying.

BACKGROUND ART

A porous ceramic article is widely used as a filter, catalyst carrier,and the like. More particularly, a porous ceramic article is used as anexhaust gas converter for a heat engine such as an internal combustionengine or for a combustion apparatus such as a boiler, a reformer ofliquid fuel or gaseous fuel, a purification system for water or sewage,and the like. Many porous ceramic articles used in the aboveapplications have a honeycomb structure to ensure a large processingarea. These porous ceramic articles are generally obtained by adding aprocessing aid such as a pore-forming agent or a binder to a ceramicpowder, kneading the powder mixture, forming the mixture in a prescribedform, and firing the formed product. The pore-forming agent is generallyused to increase the number of pores and to control the size and volumeof pores in the porous ceramic article.

Various materials functioning as the pore-forming agent have beenproposed. For example, Japanese Patent Application Laid-open No.55-100269 proposes a method for producing a cordierite honeycombstructure using starch powder. The Application discloses a method forproducing a cordierite honeycomb structure, comprising mixing 100 partsby weight of a ceramic raw material to become cordierite by firing, 1–30parts by weight of starch powder, a binder, and water, kneading themixture, forming the kneaded product by extrusion molding, and dryingand firing the formed product. The Application describes that starchpowder is added to produce pores by firing. Specifically, starch powderleaves vacant spaces (pores) after incineration during firing. Thenumerous pores thus formed allow a large amount of catalyst to adhere,ensure a sufficient catalytic effect, and remarkably increase thermalshock resistance of the honeycomb structure.

As described in the Application, starch is excellent as a processing aidto form pores. However, use of starch causes deformation, or defects inshape in many cases, of the ceramic green body in the drying step afterforming. Particularly, it may be difficult to obtain sufficientdimensional accuracy for a complicated form such as a honeycombstructure in which many cells are formed from intersecting thin walls.Therefore, to produce a porous ceramic article excelling in dimensionalaccuracy, a method of suppressing deformation in the drying step hasbeen desired.

The porosity of a porous ceramic honeycomb structure used as aninstrument for exhaust gas purification tends to further increase, sincea decrease in pressure loss is requested. Therefore, the amount ofpore-forming agent such as starch tends to increase. However, since anincrease in the amount of pore-forming agent added is accompanied by anincrease in calorific value during combustion of the pore-forming agentin the firing step, cracks may be easily formed in the ceramic article.This leads to difficulty in obtaining a nondefective porous ceramicarticle. In recent years, a foamed resin having a small calorific valuehas been preferably used as a pore-forming agent to produce a porousceramic article having a high porosity. However, if the foamed resinalone is added as the pore-forming agent for a ceramic raw materialhaving high hardness such as alumina or silicon carbide, a part of or aconsiderable amount of the foamed resin is crushed by the ceramic rawmaterial during kneading. This causes a problem such as a decreased ornonuniform pore-forming effect. Therefore, it may be difficult to obtaina porous ceramic article having a desired porosity. Accordingly, amethod for producing a porous ceramic article having a high porositywithout producing cracks or the like in the firing step has beendesired.

DISCLOSURE OF THE INVENTION

The present invention was made in view of such a situation. An object ofthe present invention is to provide a method for producing a porousceramic article excelling in dimensional accuracy by suppressingdeformation during drying. Another object of the present invention is toprovide a method for producing a porous ceramic article excelling indimensional accuracy by suppressing deformation during drying, whereinthe porous ceramic article having a high porosity can be obtained whilesubstantially no cracks are produced during firing.

The present invention provides a method for producing a porous ceramicarticle comprising a step of kneading raw materials including a ceramicraw material and a processing aid to prepare a clay body, a step offorming the clay body to prepare a ceramic green body, a step of dryingthe ceramic green body to prepare a ceramic dried article, and a step offiring the ceramic dried article to prepare a porous ceramic article,wherein the processing aid includes crosslinked starch.

In the present invention, the crosslinked starch is preferably used inan amount of 2–30 parts by mass for 100 parts by mass of the ceramic rawmaterial, and preferably has an average particle size of 2–100 μm.Further, the processing aid in the present invention preferably includesa foamed resin in addition to the crosslinked starch. The foamed resinis added in an amount of preferably 0.5–10 parts by mass, and morepreferably 1–5 parts by mass for 100 parts by mass of the ceramic rawmaterial. The foamed resin is usually formed by covering a shell wall ofresin with a water membrane. In this specification, the amount of foamedresin added refers to a mass not including the mass of water.

The average particle size of the foamed resin is preferably 2–200 μm,and more preferably 10–100 μm. The shell wall thickness of the foamedresin is preferably 0.01–1.0 μm, and more preferably 0.1–0.5 μm. Theporous ceramic article preferably has a honeycomb structure. The ceramicraw material preferably contains one or more materials selected from thegroup consisting of a cordierite raw material, mullite, alumina,aluminum titanate, lithium aluminum silicate, silicon carbide, siliconnitride, and metal silicon as the main component. The raw material claybody preferably has hardness at 95° C. of 95% or more of hardness at 80°C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic oblique view showing a honeycomb structure whichis one embodiment of the porous ceramic article produced according tothe present invention.

FIGS. 2( a) and 2(b) are schematic oblique views respectively showing ahoneycomb segment which is another embodiment of the porous ceramicarticle produced according to the present invention and a honeycombstructure of the integrated honeycomb segments, wherein FIG. 2( a) is anoblique view showing the honeycomb segment and FIG. 2( b) is an obliqueview of the honeycomb structure of the integrated honeycomb segments.

FIGS. 3( a) and 3(b) are schematic oblique views respectively showing asealed honeycomb structure, wherein FIG. 3( a) is a schematic obliqueview showing the sealed upper portion of the honeycomb structure andFIG. 3( b) is a partially enlarged view of the portion b in FIG. 3( a).

BEST MODE FOR CARRYING OUT THE INVENTION

A method for producing a porous ceramic article of the present inventionis described below in detail. However, the present invention is notlimited to the following embodiment.

The method for producing a porous ceramic article of the presentinvention comprises a step of kneading raw materials including a ceramicraw material and a processing aid to prepare a clay body (hereinafterreferred to as “clay body preparing step”), a step of forming the claybody to prepare a ceramic green body (hereinafter referred to as“forming step”), a step of drying the ceramic green body to prepare aceramic dried article (hereinafter referred to as “drying step”), and astep of firing the ceramic dried article to prepare a porous ceramicarticle (hereinafter referred to as “firing step”). The presentinvention is particularly characterized in that the processing aid addedto the raw materials in the clay body preparing step includescrosslinked starch or a mixture of crosslinked starch and a foamedresin.

If a clay body prepared by kneading the raw materials containingnon-crosslinked starch as a processing aid is gradually heated from roomtemperature, hardness of the clay body increases until the temperaturereaches around 80° C. However, hardness was found to decrease after apeak at around 80–90° C. The decrease in hardness (softening) of theclay body was found to cause deformation of the ceramic green body inthe drying step. In addition, this decrease in hardness of the clay bodywas found to exactly coincide with the change in viscosity of thenon-crosslinked starch. Specifically, the non-crosslinked starch doesnot exhibit a decrease in viscosity until the temperature reaches around80° C., but exhibits a decrease in viscosity from a peak at around80–90° C., when heated with water.

However, the crosslinked starch does not exhibit a decrease in viscosityfrom a peak at around 80–90° C. when heated with water. The clay body ofthe ceramic green body containing the crosslinked starch exhibits only aslight decrease in hardness. In some cases, the clay body exhibits evenan increase in hardness. Since a decrease in hardness of the clay bodyat around 80–90° C. is substantially suppressed, deformation of theceramic green body can be suppressed during drying. Use of thecrosslinked starch as a processing aid exhibiting the pore-formingeffect suppresses softening of the ceramic green body in the dryingstep. Therefore, a porous ceramic article with less deformationexcelling in dimensional accuracy can be obtained.

The crosslinked starch of the present invention is obtained bycrosslinking at least a part of molecules of starch to the extent thatthe crosslinked starch exhibits only a slight decrease in viscosityafter a peak at 80–90° C. when heated with water as compared with thenon-crosslinked starch and that the crosslinked starch does not causedeformation of the ceramic green body to the degree of producing adefective porous ceramic product. Preferably, viscosity of thecrosslinked starch does not substantially decrease. There are nospecific limitations to the method for crosslinking starch. A methodknown in the art may be used. Specific examples of the crosslinkedstarch include starch crosslinked with phosphoric acid, starchcrosslinked with epichlorohydrin, starch crosslinked with formaldehyde,starch crosslinked with acrolein, esterified starch, and etherifiedstarch.

There are no specific limitations to the amount of crosslinked starchadded. If the amount is too large, the calorific value during combustionof the crosslinked starch in the firing step is too large, wherebycracks may be easily produced. On the other hand, if the amount is toosmall, it is difficult to obtain a sufficient pore-forming effect. Theamount of crosslinked starch added is preferably 1–30 parts by mass, andmore preferably 5–20 parts by mass for 100 parts by mass of the ceramicraw material.

The processing aid of the present invention preferably includes, inaddition to the crosslinked starch, a pore-forming agent having acombustion temperature range different from that of the crosslinkedstarch.

Use of the processing aid including, in addition to the crosslinkedstarch, the pore-forming agent having a different combustion temperaturerange can produce a porous ceramic article having a high porosity, whilesuppressing production of cracks.

As the pore-forming agent having a different combustion temperaturerange used together with the crosslinked starch, a foamed resin with asmall resin content and a low calorific value is preferable.

If the crosslinked starch is used together with the foamed resin,production of cracks in the firing step can be suppressed, and a porousceramic article having a remarkably high porosity which is fullyappropriate for a porous ceramic honeycomb structure used as an exhaustgas converter in recent years can be produced.

The porosity can increase only to a limited extent if the crosslinkedstarch alone is used as the pore-forming agent, because thenon-crosslinked starch may easily produce cracks if added in excess ofthe above-described amount. On the other hand, if only a foamed resin isadded as the pore-forming agent to a ceramic raw material having highhardness such as alumina or silicon carbide, the foamed resin is crushedby the ceramic raw material during kneading in the clay body preparingstep, whereby the pore-forming effect decreases.

However, use of the crosslinked starch and the foamed resin incombination decreases the calorific value during combustion of thepore-forming agents, and suppresses production of cracks due to thedifference in combustion temperature between the pore-forming agents andcrushing of the foamed resin, whereby the foamed resin can exhibit itsessential pore-forming effect. Therefore, a porous ceramic articlehaving a remarkably high porosity excelling in dimensional accuracy canbe obtained. The reasons for a decrease in crushing of the foamed resinare not necessarily clear. However, it may be suggested that thecrosslinked starch has some protective effect on the foamed resin.

The amount of foamed resin added is preferably 0.5–10 parts by mass, andmore preferably 1–5 parts by mass for 100 parts by mass of the ceramicraw material. If the amount of foamed resin added is less than 0.5 partby mass, the porosity does not sufficiently increase. If more than 10parts by mass, it is difficult to solidify the clay body, wherebyprocessing efficiency decreases.

There are no specific limitations to the particle size of thecrosslinked starch or the foamed resin. However, if the particle size istoo small, the crosslinked starch or the foamed resin is filled in thevoids formed between the ceramic raw material powders, whereby thepore-forming effect decreases. If the particle size of the crosslinkedstarch is too large, formability of the ceramic green body decreases. Ifthe particle size of the foamed resin is too large, the foamed resin maybe easily crushed, whereby the pore-forming effect decreases. Theaverage particle size of the crosslinked starch is preferably 2–100 μm,and more preferably 10–60 μm. The average particle size of the foamedresin is preferably 2–200 μm, and more preferably 10–100 μm. The aboveparticle size is determined using a laser diffraction granulometer.

In the present invention, raw materials including the crosslinked starchor a mixture of the crosslinked starch and the foamed resin and aceramic raw material are first kneaded to prepare a clay body in theclay body preparing step. There are no specific limitations to theceramic raw material inasmuch as the material is a ceramic which can beformed in a prescribed form by firing or a substance to become a ceramicwith a prescribed form by firing. As the ceramic raw material, one ormore materials selected from the group consisting of a cordierite rawmaterial, mullite, alumina, aluminum titanate, lithium aluminumsilicate, spinel, silicon carbide, metal silicon, silicon nitride, andthe like can be used, for example.

The cordierite raw material is preferably the main component from theviewpoint of thermal shock resistance. The cordierite raw materialrefers to cordierite itself and/or a material that can form cordieriteby firing. The material that can form cordierite by firing comprisestalc, kaolin, fired kaolin, alumina, aluminum hydroxide, and silica, forexample, at a proportion to provide a chemical composition of 42–56 mass% of SiO₂, 30–45 mass % of Al₂O₃, and 12–16 mass % of MgO. The maincomponent herein refers to a component making up 50 mass % or more,preferably 70 mass % or more, and more preferably 80 mass % or more ofthe ceramic raw material.

As the main component, silicon carbide or a mixture of silicon carbideand metal silicon is preferable from the viewpoint of thermalresistance. When metal silicon (Si) and silicon carbide (SiC) are usedas the main component of the ceramic raw material, if the Si contentexpressed by the formula Si/(Si+SiC) is too small, the effect of Siaddition is not easily achieved, and if the Si content exceeds 50 mass%, high thermal resistance and high thermal shock resistance which arecharacteristic of SiC are not easily achieved. The Si content ispreferably 5–50 mass %, and more preferably 10–40 mass %.

The processing aid may include, in addition to the crosslinked starchand the foamed resin, one or more of other pore-forming agents such asgraphite, phenol resin, foaming resin (a resin to be foamed), polymethylmethacrylate, polyethylene, and polyethylene terephthalate. Theprocessing aid may also include one or more of binders such ascelluloses such as methylcellulose, hydroxypropoxyl methylcellulose,hydroxyethylcellulose, and carboxymethylcellulose, and polyvinylalcohol. Among these, methylcellulose and/or hydroxypropoxylmethylcellulose are preferably used, if extrusion molding is employed inthe forming step. A dispersant such as ethylene glycol, dextrin, fattyacid soap, or polyalcohol may be further added. Moreover, a liquidmedium such as water is preferably added.

There are no specific limitations to the method for kneading the rawmaterials in the clay body preparing step of the present invention. Acommon kneader, pressure kneader, uniaxial continuous extruder, biaxialcontinuous kneading extruder, vacuum pugmill, or the like may be used.The raw materials are kneaded using such a kneader to prepare a claybody. However, if a clay body prepared by kneading raw materials using acommon kneader, pressure kneader, or the like not involving a vacuumprocess is further kneaded using a vacuum pugmill, such a clay bodycontains less or no bubbles and has improved plasticity.

In the present invention, the clay body made from the raw materials hasa ratio of hardness at 95° C. to hardness at 80° C. (hereinafterreferred to as “hardness ratio”) of 95% or more, assuming that hardnessat 80° C. is 100%. Hardness ratio is more preferably 100% or more,specifically, hardness at 95° C. is over hardness at 80° C. Mostpreferably, hardness ratio exceeds 100%. These conditions furthersuppress deformation of the ceramic green body in the drying step,whereby the porous ceramic article finally produced have more increaseddimensional accuracy. In order to examine essential properties of theclay body, hardness is measured for a clay body prepared by beingfinally kneaded using a kneader involving a vacuum process such as avacuum pugmill.

The prepared clay body is formed in a prescribed form in the formingstep. There are no specific limitations to the method for forming theclay body in the forming step. Conventional methods such as extrusionmolding, injection molding, and jiggering may be used. Extrusion moldingis preferably used for forming a honeycomb structure. An extruder whichcan simultaneously prepare and form the clay body such as a biaxialcontinuous kneading extruder is preferably used to continuously performthe clay body preparing step and the forming step. In this case, theforming step is performed when the raw materials which have become theclay body in the extruder are extruded through a die.

There are no specific limitations to the form of the ceramic green bodyformed in the forming step. For example, a honeycomb structure 1 havinga structure shown in FIG. 1 is preferable. The honeycomb structure 1 hasa number of through-holes 3 passing through the structure 1 in an X-axisdirection partitioned by partitions 2 and is suitable for a filter or acatalyst carrier. As shown in FIG. 2( b), the honeycomb structure 1produced by integrating a plurality of honeycomb segments 12 canpreferably suppress production of cracks due to thermal stress.Therefore, the honeycomb segments 12 as in FIG. 2( a) are preferablyformed in the forming step.

Second, the ceramic green body prepared in the forming step is dried inthe drying step. The drying step removes moisture, liquid medium, andthe like contained in the ceramic green body. There are no specificlimitations to the drying method. Hot air drying, microwave drying,dielectric drying, reduced pressure drying, vacuum drying, or the likeis usually employed. The drying step using hot air drying and microwavedrying or dielectric drying in combination is preferable, since theentire green body can be rapidly and uniformly dried. The dryingtemperature when using hot air drying is preferably 80–150° C. to ensurerapid drying. Any of the above driers causes a temperature fluctuationto some extent, for example, in the range of 80–100° C. This temperaturefluctuation may cause deformation of the green body containing thenon-crosslinked starch. However, since the method of the presentinvention employs the crosslinked starch, deformation of the green bodycan be effectively suppressed.

Third, the ceramic dried article prepared in the drying step is fired inthe firing step to prepare the porous ceramic article. The firingtemperature and the firing atmosphere vary according to the ceramic rawmaterial. A person skilled in the art can select the firing temperatureand firing atmosphere suitable for the selected ceramic raw material. Anoxide material such as a cordierite raw material or mullite ispreferably fired in air, for example. The cordierite raw material ispreferably fired at 1, 400–1,440° C. A nonoxide such as silicon carbideor silicon nitride is preferably fired in a nonoxidizing atmosphere suchas nitrogen or argon. Silicon carbide is preferably fired at1,400–1,800° C. when bonded with metal silicon, and at 1,550–1,800° C.when bonded with silicon nitride or the like. Silicon carbide powdersmust be fired at 1,800° C. or more to bond themselves byrecrystallization. Metal silicon is preferably fired in a nitrogenatmosphere at 1,200–1,600° C. to produce silicon nitride. In this firingstep, the pore-forming agents such as the crosslinked starch burn andproduce heat. Combined use of the crosslinked starch and the foamedresin can suppress defects such as production of cracks, because of thedifference in combustion temperature and the small calorific value ofthe foamed resin.

If the porous ceramic article of the present invention having ahoneycomb structure is used as a filter such as a diesel particulatefilter (hereinafter referred to as “DPF”), as shown in FIG. 3( a) andFIG. 3( b) which is a partially enlarged view of FIG. 3( a), it ispreferable that the openings of specific through-holes 3 a be sealed atone edge 42 and the openings of remaining through-holes 3 b be sealed atthe other edge 44. In such a configuration, if the honeycomb structureis used as a filter, fluid to be processed flows into the through-holes3 b opening at one edge 42, passes through the porous partitions 2, andis discharged from the through-holes 3 a opening at the other edge 44.In this instance, the partitions 2 function as a filter and ensure alarge filtration area.

Sealing can be performed by masking the through-holes not to be sealed,providing a slurry raw material used for sealing at the opening edge ofthe honeycomb segments, and firing the slurry after drying. Sealing ispreferably performed after the forming step and before the firing stepin the method for producing the porous ceramic article, whereby thefiring step does not have to be repeated. However, sealing may beperformed after the firing step. Sealing can be performed any time afterthe forming step. The raw material used for sealing can be appropriatelyselected from the above-described preferable ceramic raw materials. Araw material the same as the ceramic raw material is preferable.

If the porous ceramic article is produced by integrating a plurality ofthe honeycomb segments 12 as shown in FIGS. 2( a) and 2(b), a pluralityof the honeycomb segments as in FIG. 2( a) and, optionally, a pluralityof honeycomb segments corresponding to the outer circumference of thehoneycomb structure, are produced, and these honeycomb segments arebound using a binder such as ceramic cement, cured by drying at around200° C., and optionally, processed by grinding in a prescribed form,whereby the honeycomb structure can be obtained. Preferable binders mayinclude a material selected from the group of the materials suitablyused as the main component of the honeycomb structure. If the differencein a thermal expansion coefficient between the binder and the honeycombsegments is too large, thermal stress is focused on the joint sectionduring heating or cooling. The difference in the thermal expansioncoefficient between the binder and the honeycomb segments at 20–800° C.is preferably 1.5×10⁻⁶/° C. or less.

When the honeycomb structure is produced by integrating the honeycombsegments, the cross-sectional area of each honeycomb segment ispreferably 900–62,500 mm², and more preferably 2,500–40,000 mm². Thecross-sectional area of each honeycomb segment of which the maincomponent is alumina or silicon carbide having a high thermal expansioncoefficient and low thermal shock resistance is preferably 900–10,000mm², and more preferably 900–5,000 mm². Irrespective of types ofhoneycomb segments, the honeycomb segments having the abovecross-sectional area preferably make up 70 vol % or more of thehoneycomb structure before circumference processing.

If the porous ceramic article produced according to the presentinvention is used as a catalyst carrier to purify exhaust gas in a heatengine such as an internal combustion engine or a combustion apparatussuch as a boiler, or to reform liquid fuel or gaseous fuel, a catalyst,for example, a metal having catalytic functions, is preferably carriedon the porous ceramic article. A method known in the art for causing acatalyst to be carried on the porous ceramic article may be used. Forexample, the porous ceramic article is washcoated with a catalystslurry, dried, and fired to cause the catalyst to be carried thereon.Examples of a representative metal having catalytic functions includePt, Pd, and Rh. At least one of these metals is preferably carried onthe porous ceramic article.

The porous ceramic article produced according to the present inventionhas internal pores. There are no specific limitations to the porosityand the pore diameter. The porosity and the pore diameter areappropriately selected according to the application.

The porosity is preferably 30–90% when using the porous ceramic articleas DPF, for example. If the porosity is less than 30%, the pressure lossis too large. If more than 90%, the ceramic article has insufficientstrength. When the porous ceramic article is used as a filter requiringa low pressure loss such as a filter which carries a catalyst thereon tocontinuously incinerate particulates, the porosity is preferably 50–90%,more preferably 50–80%, and particularly preferably 53–70%. When theporous ceramic article is used as a filter carrying a catalyst thereon,since pressure loss increases due to the catalyst carried, the porositymust be set at a high level beforehand. If the porosity is less than50%, the pressure loss in this type of filter increases. If the porosityexceeds 90%, strength of the ceramic article is insufficient. When themain component of the porous ceramic article is silicon carbide or amixture of silicon carbide and metal silicon, the porosity and thethermal conductivity are, respectively, preferably 50–90% and 5–30 W/mK,more preferably 50–80% and 7–28 W/mK, and particularly preferably 53–70%and 9–25 W/mK.

The pore diameter is preferably 2–50 μn when using the porous ceramicarticle as DPF. An average pore diameter of less than 2 μm easily leadsto an increase in pressure loss, even if a small amount of particulatesis deposited. If the average pore diameter is more than 50 μm,particulates tend to remain unfiltered.

There are no specific limitations to the thickness of the partitions ofthe honeycomb structure shown in any one of FIGS. 1–3 showing the porousceramic article produced according to the present invention. However, ifthe partitions are too thick, pressure loss is too large when fluid tobe processed passes through the porous partitions. If the partitions aretoo thin, strength of the partitions is insufficient. The thickness ofthe partitions is preferably 30–2,000 μm, more preferably 40–1,000 μm,and most preferably 50–500 μm. There are no specific limitations to thecell density (the number of through-holes per unit cross-sectionalarea). However, if the cell density is too small, strength and effectiveGSA (geometric surface area) of the honeycomb structure areinsufficient. If the cell density is too large, pressure loss is largewhen the fluid to be processed passes through the through-holes. Thecell density is preferably 6–2,000 cells/in² (0.9–311 cells/cm²), morepreferably 50–1,000 cells/in² (7.8–155 cells/cm²), and most preferably100–400 cells/in² (15.5–62.0 cells/cm²). There are no specificlimitations to the form of cross-section of the through-holes (the cellform). From the viewpoint of production, the cell form is preferablytriangular, quadrangular, hexagonal, or corrugated. The cell form of DPFof which the through-holes are alternately sealed at the edge of thehoneycomb structure is preferably triangular or quadrangular, taking thefilter area into consideration. There are no specific limitations to thecross-sectional form of the honeycomb structure. The cross-sectionalform may be circular as in FIG. 1, oval, race track-like, elliptical,polygonal such as triangular, quasitriangular, quadrangular, orquasiquadrangular, or deformed.

EXAMPLES

The present invention is described below in more detail by examples.However, the present invention is not limited to the following examples.

Example 1

To a mixture of 100 parts by mass of a ceramic raw material consistingof 75 mass % of silicon carbide and 25 mass % of metal silicon and 10parts by mass of crosslinked starch with an average particle size of 45μm, methylcellulose, hydroxypropoxyl methylcellulose, a surfactant, andwater were added and mixed to prepare a plastic clay body using a vacuumpugmill. This clay body was formed by extrusion molding to prepare aceramic green body. This ceramic green body was dried using microwavedrying and hot air drying in combination, calcinated at 400° C. in air,and fired at around 1,450° C. in an inert atmosphere of argon to preparea porous ceramic article of a metal silicon-silicon carbide compositematerial in which Si was bonded with SiC, having a honeycomb structurewith a partition thickness of 380 μm, a cell density of 31.0 cells/cm²(200 cells/in²), one side of the square cross-section of 35 mm, and alength of 152 mm.

Examples 2–10

Porous ceramic articles of a honeycomb structure were produced using theraw materials and method the same as in Example 1 except that the amountand average particle size of the crosslinked starch were as shown inTable 1. In Example 10, low-crosslinked starch was used.

Example 11

As a ceramic raw material, a mixture of talc, kaolin, alumina, aluminumhydroxide, and silica having a cordierite composition was provided. To amixture of 100 parts by mass of the ceramic raw material and 10 parts bymass of crosslinked starch with an average particle size of 45 μm,methylcellulose, hydroxypropoxyl methylcellulose, a surfactant, andwater were added and mixed to prepare a plastic clay body using a vacuumpugmill. This clay body was formed by extrusion molding to prepare aceramic green body. This ceramic green body was dried using dielectricdrying and hot air drying in combination and fired at around 1,420° C.in air to prepare a porous ceramic article of cordierite having ahoneycomb structure with a partition thickness of 300 μm, a cell densityof 46.5 cells/cm² (300 cells/in²), an outer diameter of 144 mm, and alength of 152 mm.

Examples 12–13

Porous ceramic articles of a honeycomb structure were produced using theraw materials and method the same as in Example 11 except that theamount and average particle size of the crosslinked starch were as shownin Table 1.

Examples 14–20 and 23–25

Porous ceramic articles of a honeycomb structure were produced using theraw materials and method the same as in Example 1 except that the amountand average particle size of the crosslinked starch and the foamed resinwere as shown in Table 2.

Examples 21–22

Porous ceramic articles of a honeycomb structure were produced using theraw materials and method the same as in Example 11 except that theamount and average particle size of the crosslinked starch and thefoamed resin were as shown in Table 2.

Comparative Example 1

A porous ceramic article of a honeycomb structure was produced using theraw materials and method the same as in Example 1 except thatnon-crosslinked starch was used instead of the crosslinked starch.

Comparative Example 2

A porous ceramic article of a honeycomb structure was produced using theraw materials and method the same as in Example 1 except that starch wasnot used.

Comparative Example 3

A porous ceramic article of a honeycomb structure was produced using theraw materials and method the same as in Example 1 except that thecrosslinked starch was not used and that the amount and average particlesize of the foamed resin were as shown in Table 2.

Evaluation

The porous ceramic articles obtained in Examples 1–25 and ComparativeExamples 1–3 were evaluated as follows. The results were shown in Tables1 and 2.

(1) Hardness

Hardness at 95° C. and at 80° C. of the plastic clay body prepared usinga vacuum pugmill was measured using a NGK hardness tester. The ratio (%)of hardness at 95° C. to hardness at 80° C. was determined as hardnessratio, assuming that hardness at 80° C. was 100%.

(2) Deformation

The green body in the drying step was observed by naked eye observationto identify whether or not deformation occurred. If deformationoccurred, the extent of deformation was also evaluated.

(3) Porosity

The porosity of the porous ceramic articles was measured using mercuryporosimeter. The rate of increase in porosity of the porous ceramicarticles containing the crosslinked starch as compared with the porousceramic article not containing the crosslinked starch (ComparativeExample 2) was determined.

(4) Cracks

The porous ceramic articles were observed using an optical microscope toidentify whether or not cracks were present.

TABLE 1 Type and amount of starch Amount Average Crosslinked or non-(parts particle Hardness Deformation Rate of increase in Cracks in No.Ceramic raw material crosslinked by mass) size (μm) ratio (%) duringdrying porosity by starch (%) fired body Example 1 SiC: 75% + Si: 25%Crosslinked 10 45 104 None 5 None Example 2 SiC: 75% + Si: 25%Crosslinked 2 45 103 None 1 None Example 3 SiC: 75% + Si: 25%Crosslinked 30 45 100 None 15 None Example 4 SiC: 75% + Si: 25%Crosslinked 5 2 102 None 1 None Example 5 SiC: 75% + Si: 25% Crosslinked10 10 100 None 4 None Example 6 SiC: 75% + Si: 25% Crosslinked 10 60 98None 5 None Example 7 SiC: 75% + Si: 25% Crosslinked 20 100 95 None 12None Example 8 SiC: 75% + Si: 25% Crosslinked 10 15 99 None 5 NoneExample 9 SiC: 75% + Si: 25% Crosslinked 35 10 101 None 12 PresentExample 10 SiC: 75% + Si: 25% Crosslinked 15 45 94 Small 5 None(low-crosslinked) Example 11 Cordierite raw material Crosslinked 10 45104 None 7 None Example 12 Cordierite raw material Crosslinked 10 15 101None 7 None Example 13 Cordierite raw material Crosslinked 1 15 105 None0.5 None Comparative SiC: 75% + Si: 25% Non-crosslinked 10 45 91 Large 5None Example 1 Comparative SiC: 75% + Si: 25% — 0 — 107 None 0 NoneExample 2

The porous ceramic articles of Examples 1–13 exhibited less or nodeformation in the drying step as compared with that of ComparativeExample 1. Specifically, the effect of suppressing deformation duringdrying was confirmed in these porous ceramic articles. The porousceramic articles of Examples 1–13 also exhibited an increased porosityas compared with those of Comparative Example 2. Specifically, thepore-forming effect was confirmed in these porous ceramic articles.Particularly, the porous ceramic articles of Examples 1–8 and 10–12exhibited neither deformation during drying nor cracks after firing.Moreover, the pore-forming effect by adding starch was confirmed inthese porous ceramic articles. Since the amount of crosslinked starchadded was small in the porous ceramic article of Example 13, the effectof increasing the porosity was smaller than those of the other Examples.On the other hand, cracks were produced in the porous ceramic article ofExample 9 due to a large amount of the crosslinked starch. In Example10, since hardness at 95° C. of the clay body was lower than 95% ofhardness at 80° C., the effect of suppressing deformation during dryingwas small.

TABLE 2 Particle size and amount Type and amount of starch of foamedresin Hard- Deform- Rate of in- Amount Average Amount Average ness ationcrease in Crosslinked or (parts by particle (parts by particle ratioduring porosity by Cracks in No. Ceramic raw material non-crosslinkedmass) size (μm) mass) size (μm) (%) drying starch (%) fired body Example14 SiC: 75% + Si: 25% Crosslinked 10 10 1.5 20 99 None 15 None Example15 SiC: 75% + Si: 25% Crosslinked 10 10 5.0 2 99 None 38 None Example 16SiC: 75% + Si: 25% Crosslinked 5 10 10.0 10 98 None 41 None Example 17SiC: 75% + Si: 25% Crosslinked 10 10 0.5 20 100 None 8 None Example 18SiC: 75% + Si: 25% Crosslinked 10 10 1.0 50 101 None 12 None Example 19SiC: 75% + Si: 25% Crosslinked 10 45 1.5 100 99 None 15 None Example 20SiC: 75% + Si: 25% Crosslinked 10 45 1.5 200 98 None 14 None Example 21Cordierite raw material Crosslinked 3 60 1.5 50 101 None 12 None Example22 Cordierite raw material Crosslinked 10 45 2.5 20 101 None 20 NoneExample 23 SiC: 75% + Si: 25% Crosslinked 10 10 11.0 50 98 None 44 NoneExample 24 SiC: 75% + Si: 25% Crosslinked 10 10 1.5 220 99 None 9 NoneExample 25 SiC: 75% + Si: 2S% Crosslinked 10 10 1.5 250 99 None 5 NoneComparative SiC: 75% + Si: 25% — — — 1.5 100 99 None 3 None Example 3

The porous ceramic articles of Examples 14–25 exhibited less or nodeformation in the drying step as compared with that of ComparativeExample 1. Specifically, the effect of suppressing deformation duringdrying was confirmed in these porous ceramic articles. The porousceramic articles of Examples 14–25 exhibited an increased porosity ascompared with those of Examples 1, 5, 11, and the like in which theamount and average particle size of starch were the same as those ofExamples 14–25. Specifically, the pore-forming effect was significantlylarge. Crushing of the foamed resin in the porous ceramic articles ofExamples 14–25 in the clay body preparing step was little as comparedwith the porous ceramic article of Comparative Example 3 in whichcrosslinked resin was not added. Specifically, the effect of increasingthe porosity was large in these porous ceramic articles.

When 0.5 part by mass or more of the foamed resin was added to 100 partsby mass of the ceramic raw material, the effect of increasing theporosity by the foamed resin was obtained. When 1.0 part by mass ormore, the effect of increasing the porosity further increased. However,in Example 23 in which more than, 10 parts by mass of the foamed resinwas added, processing efficiency decreased due to the difficulty insolidifying the clay body.

When an average particle size of the foamed resin was 2–200 μm, crushingin the foamed resin did not occur. The pore-forming effect was increasedin proportion to the foamed resin added. However, in Examples 24–25 inwhich the foamed resin had an average particle size of 200 μm or more,crushing of the foamed resin easily occurred. The pore-forming effectwas confirmed to decrease as the average particle size increased.

INDUSTRIAL APPLICABILITY

As described above, since the porous ceramic article produced accordingto the present invention contains crosslinked starch as a raw material,deformation during processing is small. Therefore, the porous ceramicarticle excels in dimensional accuracy. A Porous ceramic article havinga high porosity can be obtained while suppressing production of cracksduring firing by adding a foamed resin in addition to the crosslinkedstarch.

1. A method for producing a porous ceramic article comprising a step ofkneading raw materials including a ceramic raw material and a processingaid to prepare a clay body, a step of forming the clay body to prepare aceramic green body, a step of drying the ceramic green body to prepare aceramic dried article, and a step of firing the ceramic dried article toprepare a porous ceramic article, characterized in that the processingaid includes crosslinked starch that comprises at least one of starchcrosslinked with phosphoric acid, starch crosslinked withepichlorohydrin, starch crosslinked with formaldehyde, starchcrosslinked with acrolein, esterified starch, and etherified starch. 2.The method for producing a porous ceramic article according to claim 1,characterized in that the crosslinked starch is used in an amount of2–30 parts by mass for 100 parts by mass of the ceramic raw material. 3.The method for producing a porous ceramic article according to claim 1,characterized in that the crosslinked starch has an average particlesize of 2–100 μm.
 4. The method for producing a porous ceramic articleaccording to claim 2, characterized in that the crosslinked starch hasan average particle size of 2–100 μm.
 5. The method for producing aporous ceramic article according to claim 1, characterized in that theprocessing aid includes a foamed resin in addition to the crosslinkedstarch.
 6. The method for producing a porous ceramic article accordingto claim 5, characterized in that the foamed resin is used in an amountof 0.5–10 parts by mass for 100 parts by mass of the ceramic rawmaterial.
 7. The method for producing a porous ceramic article accordingto claim 5, characterized in that the foamed resin has an averageparticle size of 2–200 μm.
 8. The method for producing a porous ceramicarticle according to claim 6, characterized in that the foamed resin hasan average particle size of 2–200 μm.
 9. The method for producing aporous ceramic article according to claim 1, characterized in that theporous ceramic article has a honeycomb structure.
 10. The method forproducing a porous ceramic article according to claim 1, characterizedin that the ceramic raw material contains one or more materials selectedfrom the group consisting of a cordierite raw material, mullite,alumina, aluminum titanate, lithium aluminum silicate, silicon carbide,silicon nitride, and metal silicon as the main component.
 11. The methodfor producing a porous ceramic article according to claim 1,characterized in that hardness of the clay body at 95° C. is 95% or moreof hardness of the clay body at 80° C.
 12. The method for producing aporous ceramic article according to claim 1, characterized in thathardness of the clay body at 95° C. is 100% or more of hardness of theclay body at 80° C.